As a general rule, an instrument that measures optical properties of an object is sensitive to the polarization of light that is emitted, scattered, reflected, or transmitted by an object. The polarization sensitivity of an optical instrument may be desirable so that it can characterize the polarization of light from an object. It may also be desirable that the instrument be insensitive to polarization. Either way, a common need exists to characterize the polarization sensitivity of an optical instrument with a specified level of calibration accuracy.
One approach to characterizing the polarimetric response of an optical instrument is to place a linear polarizer between a source of known radiance and an optical aperture of the instrument; the polarizer is then rotated through one or more angles while the instrument collects data. If the transmittance and the extinction ratio of the linear polarizer are known, the polarimetric response of the instrument may be determined.
There are two shortcomings to the aforementioned approach. First, real linear polarizers are not perfect polarizers. While most of the light transmitted through the polarizer may be of desired polarization, transmittance of the orthogonal polarization of light may not be zero. The ratio of the transmittance amplitudes of the two orthogonal polarizations is called the ‘extinction ratio’. The extinction ratio may be very high, but uncertainties in its value, nevertheless, may introduce errors in the measured polarization response of the instrument. Second, uncertainty in the transmittance of a desired polarization may also introduce an error into the measured polarization response of the instrument.
Zhang et al. (Xusheng Zhang, Haoyu Wang, and Chuan He, “Analysis on the effect of extinction ratio in birefringent measurement by phase-stepping method”, Proc. SPIE Vol. 8557, Optical Design and Testing V, 2012), have developed a method for measuring the polarimetric transmittance of an optical element that is independent of transmittance and extinction ratio. This eliminates two sources of error from the measurements. This method, however, is not practical for end-to-end characterization of a remote sensing instrument. An end-to-end characterization of the remote sensing instrument requires access to its exit port located at the end of the instrument's optical path, which is typically located deep inside the instrument.
A need still exists to characterize or calibrate an optical instrument by measuring its polarimetric response to light that is emitted, scattered, reflected, or transmitted by an object. As will be explained, the present invention provides a system and a method for measuring the polarimetric transmittance of an optical instrument to a high level of accuracy by reducing two sources of errors, namely, transmittance amplitude error and extinction ratio error. As will be explained, the present invention is effective in characterizing an optical instrument without the need to know the transmittance amplitude and extinction ratio of a polarizer disposed in the optical path of the instrument under test.